Frank Cannon was originally a policeman, but he quit the force after the tragic death of his wife and infant son in an automobile accident. The tragedy drove Cannon to become a top private investigator.
Frank Cannon was a pipe smoker. He was shown with a pipe in the first two seasons but it was seen occasionally in the third and fourth season before it was subsequently dropped altogether. In reality, William Conrad was a pipe smoker.
Season one's 25 episodes began with a short scene, before the opening credits, for a preview of what would be televised, that hour. Last four seasons and 96 episodes began normally with the opening credits, no preview of what would be seen.
In most episodes, the license plate for Frank Cannon's "Cannonmobile" was 963-XUJ. In others it is 825-DQI
In the main title and background music, the Cannon melody is usually played by the tuba and/or trombones as the composer (John Carl Parker) alludes to Frank Cannon's girth.
According to the book "Creating the Filmation Generation", Lou Scheimer was interested in producing an animated spin-off called "Young Cannon" in the 1972-1973 season. They got the rights from Quinn Martin, but Fred Silverman at CBS didn't buy it